1. Field of the Invention
The present invention relates to an on-board vehicle automatic sound volume adjusting apparatus for use with on-board vehicle stereo equipment.
2. Description of the Related Art
Inside a running vehicle, the running noise masks the sound reproduced by speakers of on-board vehicle stereo equipment, with the result that the sound volume is perceived lower than the actually reproduced volume level. This impediment to satisfactory listening is conventionally circumvented by an automatic sound volume adjusting apparatus which the automatically adjusts the sound volume level in response to the running noise level being detected inside the vehicle.
With such prior art on-board vehicle automatic sound volume adjusting apparatus, as shown in FIG. 1, an input audio signal supplied from the signal source such as a cassette deck, not shown, is output via an electronic rheostat 5 (i.e., VCA). The input audio signal level is detected by a signal detection circuit 1. The output signal level of a microphone 2 that picks up the running noise is detected by a noise detection circuit 3. The difference in output signal level between signal detection circuit 1 and noise detection circuit 3 is detected by a subtracter 4. An output signal from the subtracter 4 becomes a control signal of the electronic rheostat 5. Thus, the level of attenuation electronic rheostat 5 is controlled in accordance with the control signal level.
With the prior art on-board vehicle automatic sound volume adjusting apparatus of the above-described construction, the level of attenuation of the electronic rheostat 5 is lowered as the running noise level is raised by increased vehicle speed. The sound volume level goes up accordingly. The higher the running noise level becomes, the greater the average volume of the reproduced sound from the speakers becomes. However, the reproduced sound volume being masked due to the running noise is lower than a minimum value "pp" shown in FIG. 2. At that minimum volume value, the masking is not canceled even as the sound volume levels goes up in response to the rising of the noise level. On the other hand, a maximum volume value "ff" of the reproduced sound is not masked by the running noise. Thus, there is a problem such that the sound volume becomes too high in response to a rise in the running noise level.